Patent GB-931743-A (GB)
Title: Pneumatic pressure transmitting and measuring devices for producing an output signalindicative of an investigated quantity
IPC: F15C4/00
Filed: 1959-08-06
Abstract: 931,743. Pressure gauges. BAILEY METER CO. Aug. 6, 1959 [Sept. 9, 1958], No. 26986/59. Class 106 (2). [Also in Groups XXVI and XXIX] A pneumatic pressure transmitting and measuring device comprises a movable beam sealingly extending through a casing housing a flexible structure responsive to pneumatic pressure which applies to the beam a force proportional to the pressure to control the outlet pressure from the nozzle of a nozzle-baffle coupling between the beam and a pneumatic amplifier to establish an amplified output signal, pressure-actuated means responsive to the signal to apply a restoring force to the beam and means connecting the beam to a fixed support to inhibit axial movement of the beam and to bias it into engagement with the pressureactuated means. The flexible structure subjected to a differential pressure on opposite sides of a restriction 12, Fig. 1, in a flow line 10, comprises a diaphragm unit 18, having stainless steel diaphragms 104, 106 secured to an annular member 94 to define a sealed space containing a non-compressible liquid, a spacer 107 and diaphragm reinforcing plates all secured together by a screw 108 so that the diaphragms operate as a unit. A link 40 connects the unit to a force beam 42 pivoted about a metal diaphragm 52, Fig. 2, clamped between the diaphragm casing 16 and a plate 48 supporting the frame 54 of a pneumatic pressure transmitting device 14. The beam 42 consists of two offset parts screwthreaded together and carries on a bracket 59 a flexible plate 58 which coacts with a leak-off nozzle 60 connected by a pipe 62 to an expanding chamber in a reset amplifier 28. The amplifier is connected to a compressed air source, a recorder 38 and through a selector station 32 to a diaphragm-actuated valve 34 to control flow in a line 36 in accordance with the rate of flow in the line 10. The line 36 may be an extension of the line 10 or part of a separate flow system. The recorder 38 produces a continuous record of the flow rate. The amplifier 28 is also connected to a restoring bellows 76 anchored at one end to a plate 77 connected to the frame 54 and at its other end to a channel-shaped restoring beam 65 which through an adjustable fulcrum 75 engages the beam 42 to restore the beam and hence the baffle-plate 58 to their initial positions when the output signal corresponds to the input signal. The beam 65 pivotally mounted on a hinge spring 68 carries a screw-threaded rod 70 held against axial movement which on rotation adjusts the position of the fulcrum. A spring 84 connected between the frame 54 and the beam 65 permits adjustment of the zero flow rate output control signal from the amplifier 28. To overcome the output signal error introduced within the diaphragm unit casing 16 urging the beam 42 upwardly, flexure strips 86 are anchored at their bottom ends to the plate 48 and at their upper ends between a U-shaped clamp member 87 and the beam 42 to inhibit axial movement of the beam. These strips are also given an inherent bias so that the beam 42 and fulcrum 75 are maintained in contact with each other at low flow rates. The bias and the loading of the spring 84 then being adjusted so that their effect on the beam 65 substantially cancel each other. To compensate for variations in temperature the bracket 61 which carries the nozzle 60 is made from material having the same coefficient of thermal expansion as the plate 48 and its length together with its spacing from the rear wall of the frame 54 is made equal to that portion of the plate 48 subjected to thermal expansion so that the nozzle 60 and baffle 58 retain the same relative positions with respect to each other. The device may be used to measure level in an open tank 122, Fig. 8, or a closed tank 128, Fig. 9. In the former, the left-hand side of the diaphragm unit 18 is connected to atmosphere and in the latter by a conduit 130 to the space above the liquid. In the latter case the conduit 130 is maintained full of liquid by suitable means so that the higher of the two pressures acting on the diaphragm unit occurs on its left-hand side, this pressure remaining constant. In each case a suppression spring 120 is fitted between the U-clamp 87 connected to the beam 42 and the plate 77 to which the restoring bellows 76 is anchored. The spring 120 in the former case is adjusted so that it applies a force to the beam 42 to suppress the force applied thereto due to the difference in elevation between the tank 122 and the device 14. The spring 120 in the latter case is adjusted so that the maximum level in the tank 128 corresponds to the minimum differential pressure acting on the diaphragm unit. The spring 120 is easily detachable when not required. A motion balance square root extracting mechanism may be added to the transmitting device 14 shown in Fig. 2, to provide a linear output signal. In this instance the plate 77 and bellows 76 are removed as a unit and replaced by a plate 140, Fig. 10, which carries on hinge springs 144 a two-armed lever 142. The lever at one end is attached to the movable end of a bellows 150 and a spring 148 acting on the restoring beam 65 and at its other end engages a rod 152 spring-mounted on a lever 160 pivotally mounted on an extension of the plate 140 by hinge-springs 163. The rod 152 positions a baffle 154 with respect to a nozzle 156, both being mounted on the lever 160. The output control pressure of the amplifier 28 in this instance only effects expansion of the bellows 150. A separate amplifier is fitted, the input of which goes to the nozzle 156 and the output to the conduit 30, Fig. 1, and a bellows 162 mounted on the lever 160 to restore the relationship between the baffle 151 and the nozzle 156. The pivot axes of the levers 142, 160 are arranged to give a cosine relationship between the motions of the levers 142, 160 to achieve the desired object of extracting from the added amplifier an output signal which bears linear relationship with the measured flow rate.